The four Elements of astrology refer to four basic temperaments: Fire, Earth, Air, and Water. 

They are also called Triplicities.  These vibrational patterns have a connection to every third sign of the zodiac, which form harmonious trine aspects to each other.

Webster's dictionary defines the word temperament  as a "combination of mental, physical, and emotional traits of a person; the natural predisposition".

And like many other human traits, the expresion of the Elements can range from mild to extreme.

Fire is associated with heat and warmth, the radiating energy of life itself.  The energy of these signs is enthusiastic and strong, even overpowering at times. 

These are the signs associated with putting the physical world to practical use.  The earth signs are especially tuned into the physical senses.  Patience, persistence, and self-discipline are the forte of the earth signs. 

The air signs correlate with mental perception and expression, detached from emotion and material relevance.  Their focus is the transmission of ideas.  When they get too abstract though, someone needs to help them get their feet back on the ground.

The sensation of water is cooling and healing, yet it is also powerful and permeating.  What these signs have in common is their deep, intense emotions.